1. Field of the Invention
The present invention relates to the technical field of TV tuners and, more particularly, to a method and apparatus of automatically selecting audio format for output signal of a tuner in a television system.
2. Description of Related Art
Currently, the popular formats for television (TV) signal include NTSC, PAL, SECAM, and so on. The NTSC format is also referred to as an N system, which is a color TV broadcasting standard defined by US National Television System Committee (NTSC) and essentially has two branches, NTSC-J and NTSC-US (also referring to as an NTSC-U/C).
The NTSC format has a frame rate of 29.97 frames/sec., 525 scan lines, a interlaced scanning mode, an aspect ratio of 4:3, and a resolution of 720×480. The NTSC format solves the compatibility of color broadcasting and monochrome broadcasting; however, the phase distortion and unstable color presentation exist in the NTSC format.
FIG. 1 is a schematic view illustrating the NTSC TV system parameters in the prior art. The letter M indicates monochrome standard for lines and field rates (525/59.94), a video bandwidth of 4.2 MHz, an audio carrier frequency of 4.5 MHz above the video carrier frequency, and an RF channel bandwidth of 6 MHz. In addition, the NTSC refers to the technique of adding color information to the monochrome TV signal.
The NTSC4.43 format is commonly used for multi-standard analog VCRs. The horizontal and vertical timings FH, FV of an NTSC4.43 image signal are identical to those of an NTSC-M image signal. The NTSC4.43 image signal has the same encoding as a PAL modulation format and a color subcarrier frequency of 4.43361875 MHz.
The NTSC-J format is used in Japan, and its horizontal and vertical timings are identical to those of the NTSC-M TV scheme.
In the 1950's, a color TV broadcasting was planning in Western Europe. In order to avoid the defects of the earlier developed NTSC TV signal format, including a color tone shifting being likely to occur at a poor receiving condition, different color TV formats suitable for Western Europe was developed, which are the PAL and SECAM formats TV image signal. FIG. 2 is a schematic view illustrating PAL TV system parameters in the prior art. The PAL and SECAM formats both have the same image frequency of 50 Hz to meet with the 50 Hz AC power used in Europe.
The PAL format was developed by the Telefunken company in West Germany in 1963. The BBC firstly applied the PAL format at a field trial of the BBC2 channel in 1964, and entirely used the color TV broadcasting to all channels in 1967. In 1967, the PAL format was first used for broadcasting on a color TV system in West Germany. In 1998, The ITU has defined the PAL format as “Recommendation ITU-R BT.470-6, Conventional Television Systems” formally.
The SECAM format indicates “sequential color transfer and store”, which was developed successfully in France in 1966. For a signal transfer, a brightness signal is sent line by line, and two color difference signals in interleaved lines are sent sequentially; i.e., scanning the lines in interleaved can separate the transmission time so as to avoid the cross color artifact and associated color distortion in transmission. The SECAM format has higher interference tolerance and better color effect, but poor compatibility. The SECAM format uses a frame rate of 25 frames/sec., 625 scan lines in interleaved line scan, an aspect ratio of 4:3, and a resolution of 720×576.
Since different nations have different TV signal formats, decoding of the video and audio signals in a TV image signal becomes difficult. In known patents, U.S. Pat. No. 6,765,621 granted to Okada for a “Color system discriminator” has disclosed a frequency detector which only detects whether an audio signal is presented at 4.5 MHz, but not disclosed how to distinct a color subcarrier frequency presented at 4.43 MHz from the audio signal presented at 4.5 MHz.
When a tuner supports the NTSC358-M/BG/DK/I, NTSC443-BG/DK/I, PAL-M, PAL-BG/DK/I, and PAL-N formats, the color subcarrier frequencies of video signals of the NTSC443-BG/DK/I and PAL-BG/DK/I signals are around 4.5 MHz, and in this case an input signal around 4.5 MHz cannot be detected as a video or audio signal by directly comparing the amplitudes of signals around 4.5 MHz.
Therefore, it is desirable to provide an improved method and apparatus of automatically selecting audio format for output signal of a tuner in a television system, so as to mitigate and/or obviate the aforementioned problems.